Doesn't he die when I attack him directly?? Does anyone have the same problem? I attacked him with summoner of illusions + secret pass + unon attack. He didn't lose a single LPoint.
Read the Union Attack card definition - the monster equipped with Union Attack cannot inflict battle damage. That's why every Union Attack deck uses Gravekeeper's Vassal since its card effect is battle damage = effect damage (you get from leveling up Ishizu to lvl. 3)
To user 'Farm Not Work':
The reason why it doen't allow you to use Secret Pass on GV is because his mammoth's effect is: 'Monsters your opponent controls cannot attack the turn they are summoned.' That means that you need to attack with vassal the next turn he is summoned. Cheers.
Here's the little tips, summon your Vassal on your 2nd last turn if there's a Mammoth on Pegasus's field. Make sure you have 1 defensive trap (windstorm/anubis). So your Vassal can attack on the last turn.